Perhaps the quest will require the recovery of a quantity of things instead of one. Perhaps a dungeon could have 10 quest items in it and the quest requires the the PC retrieves 5 of them. Who knows. Maybe the dungeon in question was the home of a particular people. To complete the quest you must recover 5 examples of armor or weapons. Perhaps you are paid more or gain notoriety for recovering more.

Ok so on my side I have tested this since I started to play for a few dozens of hours

For most of dungeons there were not too many issues, I could finish most of them in less than 30 min and find quest items/targets

I only remember of one specific dungeons where for some off reason it was "separated" in two different parts, with the quest target being in a part that was unaccessible. The dungeons was small so I was able to browse all walls to look for a hidden door but no luck. THe only way I managed to get there was with a cheat command.

So I would say it mostly works with maybe a few exceptions. How do we know for sure the blocks are all connected ?

So I would say it mostly works with maybe a few exceptions. How do we know for sure the blocks are all connected ?

With smaller dungeons enabled there is only a single central block, so by definition it will be connected to itself. Not all blocks allow mundane traversal however. Sometimes it is necessary to swim or fly, or solve a teleport puzzle, or manipulate moving parts to traverse fully.

If you can give me an example of a dungeon and the explain the difficulty you had, I can take a look at that block with smaller dungeons enabled to see if it's actually viable or not. It's certainly possible not all blocks in game data are self-viable. If that's the case, these blocks can be blacklisted from smaller dungeon selection.

Yep - floating skulls, red brick doors, pressure plates on the floors, tapestries, secret doors, moving walls, and more. There are loads of tricks built into various dungeons for getting around. Whenever you hit a seeming dead end, there's usually a secret nearby. The main problem is that DF doesn't help you find any of them.

DFU uses the same game data as classic. All of these little action records were put into the game by the original dungeon designers who built the block layouts back in the day.